This invention relates to a steam turbine and, more particularly, to a device that enables adjustment and optimization of turbine wheel steam balance hole area for existing wheel steam balance holes in retrofit applications to facilitate reuse of an existing rotor.
Steam balance holes are used to optimize the flow and performance of steam turbine stages. Generally, a plurality of steam balance holes are used to pass leakage flows across the turbine wheel to minimize the disruption of the main stage flow and/or to provide cooling to the wheel. For a typical stage, if the total hole area is too small, steam will enter the main steam flow path between a rotating blade and the stationary blade. This intrusion flow disrupts the main flow and causes losses that are detrimental to stage performance. If, on the other hand, the total steam balance of the hole area is too large, flow is sucked from the main steam path, thereby decreasing the work generated by the stage.
In retrofit applications, where the steam path (rotating and stationary blades) is being replaced, the rotor (including the turbine wheels and shaft) is often the most expensive component, and often takes the longest to produce. The on-going development of brush seals to seal the shaft leakage and the optimization of radial clearances has generally caused a reduction in the required steam balance hole area. As a result, in a retrofit situation, the steam balance hole area is usually far greater than required for optimum stage performance, and therefore, the advantage of modern leakage control may not be fully realized. The requirement for a smaller steam balance hole area, coupled with the economic advantage of reusing the existing rotor, indicates the need for a device to adjust and optimize the existing wheel steam balance hole area during a steam path retrofit.